Radiant Rig XL does not work with my character mesh because there is a script that interferes with it
Here were the steps I took to diagnose the problem you are having:

1) When I opened your file, I saw the error message "Auto-run disabled: Text 'rig_ui.py'" (see screen shot 1)


2) Near the Help item at the top, I switched the layout from 'gp2' to 'Scripting' (see screen shot 2)


3) I saw a script which is designed to modify the rigify plugin. Radiant Rig depends upon Rigify as stated in the instructions. If you are changing Rigify, I can't predict what will happen. I did not have time in the morning to debug the rig_ui.py attached to your file. (see screen shot 3)



4) I selected the figure in your file and merged all the parts into one object (CTRL+J)

5) I exported your object.

6) I created a blank blender file; I was able to import the exported figure into this new file.

7) I ran my script and it fitted the mesh better.

Please follow those exact steps.